Denver is still one of the worst cities when it comes to unhealthy levels of ozone pollution (read: smog), finds a new report from the American Lung Association (ALA). Although Denver did better than last year—when it was the 8th-worst in America—this year’s 11th-place finish is still cause for concern. (Fort Collins came in 15th this year, down from 10th place last year.)

The ALA’s study measures multiple types of pollution in hundreds of American cities. Its data comes from the Environmental Protection Agency’s (EPA) Air Quality System, which tracks air quality across the U.S. Current smog levels in Denver mostly affect what the ALA calls “sensitive groups,” such as children, older folks, those with respiratory or cardiovascular diseases, and others. The ALA reports that hundreds of thousands Denver residents fall under this category.

The EPA cites industrial facilities, electric utilities, and motor vehicle exhaust as primary contributors to smog. Colorado Public Radio reported in 2015 that Denver’s ozone level is sometimes worsened as wind carries ozone pollution from other cities into Denver.

A helpful resource for those concerned in the Mile High City is the Air Quality Index on AirNow. The tool tracks daily air quality in Denver, and tells residents when they might need to alter their routine. On bad days, for example, those who are vulnerable to polluted air should limit outdoor activities to the early morning and night, when smog levels are lower. Here’s how you can do your part to limit contributions to Denver’s pollution problem: